Tuesday, February 2, 2010

Pot-bellied Pteranodon

Way back in 2008, I was working on a skeletal reconstruction of Pteranodon longiceps, sourced mostly from Chris Bennett's (1991) dissertation. It included a dorsal and a lateral view, and all I had left to do were the feet (feet are fiddly, annoying, and somewhat boring). While I was procrastinating about the feet, Mike Hanson beat me to the punch, and produced a skeletal reconstruction based on the same source. It wasn't just that he got there first, but it was better than mine to boot, with more views, and a more carefully drawn skull. So I gave up, and left my still footless Pteranodon to go mouldy on my hard drive.

A couple of weeks ago, I decided to revisit my Pteranodon skeletal, to see if it was in a fit state to be incorporated into a new project. I also wanted to include the suggestion made by Claessens et al (2009) that the sternum sloped ventrally, rather than dorsally or flat and usually shown. As I started tweaking things, especially as I was working on an anterior view, I realised there was something quite wrong with my Pteranodon… the scapulocoracoid, when drawn in anterior view, was dorsoventrally way too short. I couldn’t get the coracoid to articulate with the sternum, not by a long shot.

My drawing matched Bennett’s (1991), Hanson’s (2008), Claessens et al (2009), and other drawings pretty well. So, if mine was wrong, then there was something wrong with a lot of the Pteranodon reconstructions out there.

Moving the sternum up posed something of a problem, however, because the ribs didn’t seem to fit. Looking into this more closely, it appears Bennett’s drawings show the ribs flattened toward the viewer. In other words, there is little accounting for curvature and foreshortening, which has the effect of making the ribcage far too deep.

Comparing different people's drawings, something quite interesting emerged, Pteranodon is nearly always reconstructed with a teardrop shaped trunk. Bennett’s original 1991 drawing and Hanson’s closely related 2008 drawing both have the error of an overly deep ribcage and scapulocoracoid. They also have the sternum sloping dorsally. These things combined give a strong teardrop shape. Greg Paul’s (2002) drawing has a similar shape, but it achieves this in a completely different way.* Paul seems to have the depth of the ribcage and scapulocoracoid right, and even has a gentle ventral slope to the sternum, but he also adds a deep keel to the sternum, which I presume is meant to be cartilaginous (it isn’t preserved in pterosaurs I’ve seen). So again, a classic teardrop shape. Claessens et al (2009) has a more pronounced ventral slope to the sternum, but it also has the overly deep ribs and scapulocoracoid; which makes for a slightly more portly teardrop.

If we combine a (keel-less) ventrally sloping sternum, and make the ribcage and scapulocoracoid shallower as I think they ought to be, we end up with a different sort of shape for Pteranodon's trunk. A distinctly pot-bellied one:

Sloping the sternum ventrally would make most pterosaurs deeper in the mid-trunk than we are used to drawing them, so I am looking forward to seeing whether other pterosaurs will turn out to be so hilariously shaped.

Just curious, are you short a few (3) vertebrae? From what I've seen there should be 12 pairs of dorsal ribs and the first sternal rib (10th vert from the skull) actually articulates with the sternum with sternal ribs connecting the others as you've indicated.